Well, I'm pretty sure I've finally
sorted this issue now as I've been driving around, plume free for some time now.
A few weeks ago I decided to get my injectors cleaned as they'd never been done to my knowledge. I also pressurised the inlet manifold with a small compressor I 'liberated' from my work (was away to go into the skip!). Its only a very small one, about the size of an alternator so can't generate high volume. Even though its small, I could hear hissing coming from the back of the engine. I felt around the birds nest of boost pipes round the back of the inlet and couldn't feel anything but, I could feel a slight cold draught on my hand so I knew I was close. It turned out the metal gasket on the plenum was leaking!
I took off the plenum and attached piece of inlet manifold, leaving the throttle bodies on the head. It turned out that someone had the plenum off in the past, probably with the engine in-situ and therefore couldn't tighten up two of the cap screws, which were barely finger tight.
So I replaced the gasket and fitted a couple of jubilee clips to some of the boost pipes. Result...still smoking under boost?! WTF!!
That was the last straw. The only thing it could possibly be now is the fuelling. I took the decision to lower the fuel pressure by 1psi and see what happened...less smoke! So I've been incrementally playing with the fuel pressure since and have found 'the sweet spot' where it doesn't smoke on boost but if I turn up the pressure by 1psi it does
smoke (although nowhere near as bad as before).
TBH I can't really understand it. I got the fuel pressure gauge checked at work and was told it was accurate at 3bar to within 1psi so, that's what I've been using to set the pressure all this time. The guy who checked the gauge checks calibration on most of our gauges in-house all the time and has the proper equipment.
So the outcome is that I've now got my fuel pressure set at 40psi (according to the gauge) instead of the 'normal' 43/44psi I used to run it at.
One other thing I did while the injectors were away being cleaned was to soak the tops of the pistons in "marvel mystery oil" for four days. The thinking behind this was it might help clean some of the crap off the pistons and rings. Various videos I've watched certainly indicated it was good at this.
Well, it worked a treat! it didn't do much to the piston crowns but, more importantly it seems to have cleaned/unstuck the rings. It wasn't using that much oil before but its barely used 1/4 of a liter in the past month since using it. I've also poured some into the oil to gently clean the internals before its next oil change. I'm actually pretty impressed by it!
The car now feels quicker than before, especially at 3.5/4k rpm on the dual carriageway in top gear but more importantly, no more smoking!
Its been a long and f'king frustrating journey but I finally got there...just!
Cheers for all the help and assurances guys, it would've been a smoking shell if it wasn't for you lot!